Water mass transformation in the North Atlantic over 1985?2002 simulated in an eddy-permitting model

• Main Authors: Marsh, R., Josey, S. A., Nurser, A. J. G., de Cuevas, B. A., Coward, A. C.
• Other Authors: Southampton Oceanography Centre
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2005
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Labrador Sea; north atlantic current; North Atlantic; Northeast Atlantic
• Online Access: https://hal.science/hal-00298377; https://hal.science/hal-00298377/document; https://hal.science/hal-00298377/file/osd-2-63-2005.pdf

International audience Water mass transformation in the North Atlantic is examined in an eddy-permitting simulation with the OCCAM ocean general circulation model, forced by realistic surface fluxes over the period 1985?2002. Three regions are considered: the Subtropics, the Mid-latitudes and the Northeast Atlantic. The oceanic boundaries of each region coincide with hydrographic sections occupied in recent years. These regions broadly represent the formation sites of Subtropical Mode Water (STMW) and Subpolar Mode Water (SPMW). A water mass budget is obtained for each region and year. Terms in the budget comprise surface-forced transformation rates, boundary exchanges and unsteadiness. Unsteadiness is relatively small, so that regional net water mass transformation is largely balanced by net boundary exchanges. Transformation rates due to mixing are then obtained as the difference between net and surface transformation rates. Transports at the boundaries are compared with recent observations, and reasonable agreement is obtained. For the period 1985-1993, model surface transformation rates are broadly in agreement with equivalent rates computed using the globally-balanced SOC fluxes of heat and freshwater, derived from ship observations. In each Atlantic region, surface transformation rates reach 10-15 Sv, based on both model and SOC fluxes. Higher spatial and temporal resolution in the OCCAM surface fluxes may improve the realism of surface transformation rates in some regions, notably the Labrador Sea. However, the unrealistic location of the North Atlantic Current too far south leads to spurious surface heating east of the Grand Banks in OCCAM. Period-mean transformation rates due to mixing reveal the formation of intermediate waters in each region due to the "consumption" of lighter and denser waters formed by surface fluxes, and are comparable with recent inverse estimates. There is, however, strong interannual-to-decadal variability in the consumption rates. In the subtropics, STMW consumption rates ...